The invention relates to an arrangement for and method of determining the position of a marker in an organic cavity, in particular for determining local passage speeds of a marker while passing the gastro-intestinal tract, particularly the small bowel area. The invention is applicable within the frame of medical diagnosis of the gastro-intestinal tract and does not concern a diagnosis procedure per se.
There are medical examinations known which require a repeated detection of the local passage speeds of a marker while passing the gastro-intestinal tract. This is the case, for example, with chronic inflammatory intestinal diseases such as Morbus Chron, functional gastro-intestinal diseases, and in physiological examinations of the gastro-intestinal tract. The conventional diagnosis techniques such as X-ray examination under use of contrast meals are not applicable in such cases due to the radiation exposure. The same comes true for scintigraphic methods.
A radiation exposure is avoided with known techniques such as the nuclear spin tomography [M. Reiser, W. Semmler (editor) "Magnetoresonanztomographie", Springer press, Berlin/Heidelberg, 1992] the sonography [M. Amend, C. Jakobeit, L. Greiner, Verdauungskrankheiten 13 (1995), No. 1, pg. 21], the use of metal detectors [K. Ewe, Therapiewoche 41 (1991), pg. 77], the inductive detection of soft magnetic tracers [Y. Benmair, B. Fischel, E. H. Frei, T. Gilat, The American Journal of Gastroenterology 68 (1977), pg. 170], and the local position detection of permanent magnetic markers [L. Trahms, R. Stehr, J. Wedemeyer, W. Weitschies, Biomedizinische Technik 35 (1990), pg. 158].
The nuclear spin tomography or the magneto-resonance is an expensive method which is not suitable for examinations which have to be repeated very often, apart from being too slow for the detection of local passage speeds, which require time intervals in an order of size of 10 s. for the successive position detection of markers [K. Fitzgerald, IEEE Spectrum 27 (1990), pg. 52].
Sonographic examinations have not been employed for the detection of local passage speeds up to now, but only for measuring general transit times of larger sections of the gastrointestinal tract [M. Amend, C. Jakobeit, L. Greiner, Verdauungskrankheiten 13 (1995), No. 1, pg. 21], since air volumes in the abdominal cavity cannot be penetrated by ultrasound and will result in a faulty position detection of the marker. Such deficiencies could be reduced by completely filling the bowel with a liquid, however, a filled bowel is not suited for a diagnosis due to the changed peristalsis.
It is feasible to determine the position of metal particles by metal detectors. However, the lateral accuracy of the position detection decreases with the increasing distance from the body surface and is worse than 1 cm at a distance of &gt;10 cm [K. Ewe, Therapiewoche 41 (1991), pg. 77]. Said paper does not report of the accuracy of depth-measurements. Since the accuracy of depth-measurements is generally worse than the lateral accuracy this method is insufficient for local passage speed measurements.
The accuracy of position detection obtained with an inductive measurement of a soft-magnetic tracer satisfies examinations of the gastric contents decrease per unit time of a soft-magnetic meal having an initial volume of more than 100 cm.sup. 3 [Y. Benmair, B. Fischel, E. H. Frei, T. Gilat, The American Journal of Gastroenterology 68 (1977), pg. 170]. However, on the one hand, a measurement of the local passage-speed in the bowels is not feasible, since the large test meal volume uncontrollably distributes while passing the bowels. On the other hand, the test volume cannot be reduced substantially since otherwise the secondary magnetic field produced by the tracer, even when highly compensated, will become so small that it cannot be separated from the residual signal of a primary magnetic field applied during measurement.
Furthermore, it is known to magnetically charge-up permanent magnetic markers before being administered to a patient [W. Weitschies, J. Wedemeyer, R. Stehr, L.Trahms, IEEE Trans. Biomed. Eng. 41 (1994), pg. 192]. However, the detection of the position of the markers via their secondary magnetic field is considerably affected by interference fields (for example, by the magnetic field of the earth) so that the measurements have to be carried out in an extremely magnetically screened special chamber. Even then this method is not suitable for a detection of the local passage speed in the entire gastro-intestinal tract. Hence, a position detection is only feasible in the stomach and in the large bowel, due to the transversal and rotational movements of the marker, and even in these ranges, where the retention time of the marker is comparatively high, the accuracy is insufficient.